This invention relates to electronic circuits and more particularly to the demodulation of frequency modulated (fm) radio signals.
Whereas electronic circuits for the demodulation of fm signals have been known since the use of fm for radio broadcasting began in the nineteen thirties, there has been a major problem with all existing demodulators, namely, that if two fm transmissions reach the receiver at the same carrier frequency and amplitude, mutual destruction of both transmissions occurs and all intelligibility is lost.
This is a fundamental property of fm demodulators which use saturating circuits prior to the fm demodulator to remove amplitude noise effects from disturbing the wanted signal.
This effect is called co-channel interference especially when the second interfering transmission is a different station. There are four other types of similar interference which result in complete loss of carriers of similar strength (1-15 dB difference) and having the same or similar carrier frequency.
It is an object of the present invention to provide a method and apparatus for minimising and, in some cases, eliminating the aforementioned problems from fm demodulators.
According to the present invention there is provided a method for the demodulation of fm signals, the fm signal, comprising a wanted carrier signal and an interfering carrier signal, the method comprising the steps of: generating a first signal, said first signal being a function of the ratio of the interfering carrier to the wanted carrier and of the instantaneous difference between the frequency of the wanted carrier and the interfering carrier; generating a second signal from said first signal by removing any dc value from said first signal; generating the mathematical square of said second signal; generating a dc value from said squared signal; generating a numerator function being a function of said dc value, said second signal and a dc offset; generating a denominator function being a function of said first signal and said dc offset; performing a division process between said numerator function and said denominator function to generate a quotient function; and using said quotient function to control a demodulator circuit to generate a desired output.
Further according to the present invention there is provided a circuit for demodulating fin signals, the fm signals comprising a wanted carrier signal and an interfering carrier signal, the circuit comprising means for generating a first signal, said first signal being a function of the ratio of the interim carrier to the wanted carrier and of the instantaneous difference between the frequency of the wanted carrier and the interfering carrier; means for generating a second signal from said first signal by removing any dc value from said first signal; a squaring circuit for generating the mathematical square of said second signal; means for generating a dc value from said squared signal means for generating a numerator function being a function of said dc value, said second signal and a dc offset; means for generating a denominator function being a function of said first signal and said dc offset; a division circuit for performing a division process between said numerator function and said denominator function to generate a quotient function; and a demodulator circuit conditioned by said quotient function to generate a desired output.